Big overhaul replacing selenium by puppeteer. This does not work for now due to
problems with SSL being a requirement for external endpoints.

Make it easier to override the default command.

fix #61

Change base image to node:8-alpine

Add docker-compose.yaml file

- Add ports, volumes, env_file and environment options
- Update the .gitignore to add .env file
- Access URL for the external services now uses docker-compose hostnames.
- selenium-server is now a container service dependency

Selenium is no longer directly managed by directly by hublot, but is instead an
external service dependency.
This dependency is managed in docker-compose with a selenium server image.

This simplifies a LOT of things and make the project organization cleaner.

The build time has been reduced,
and the Dockerfile now respects the best practices.

Add the script xvfb-chromium
Change start.sh to launch xvfb-chromium, and export the right value of DISPLAY
Change also the base image of the Dockerfile (node:8-strecht)

The base image is now node:8 (and no more node:8-alpine).
Changed the way Java 8 is installed.
Install other packages (chromium, xvfb, libgconf-2-4).

The tmp directory is now directly created in the webdriver-start script.

Node version has been changed to 8-alpine because Java 8 was not available in node:7.10